Racket string and method for producing the same and racket strung with the same

ABSTRACT

A racket string  10  of the present invention includes a synthetic fiber. The cross section is flattened by heating, compressing, and deforming the string after it has been formed, and an indentation is made in any part of the string. The flattened string is wound in a substantially non-twisted state. A method for producing the racket string includes heating the string at a temperature of Tg (° C.) or more and Tm −10 (° C.) or less, where Tg (° C.) represents a glass transition point and Tm (° C.) represents a melting point of a main synthetic fiber constituting the string, compressing and deforming the string between rollers  7   a  and  7   b  that are arranged with a predetermined clearance therebetween; and then cooling and winding up the string. In a racket strung with the racket string of the present invention, a principal surface of a hitting surface of the racket is formed of the flat surfaces of the string, and the strings include uneven portions due to torsion that are present partially and non-uniformly on the hitting surface of the racket. Thus, the present invention provides a racket string that can make full use of the features of the flat surface, a method for producing the racket string, and a racket strung with the racket string.

TECHNICAL FIELD

The present invention relates to a string for rackets of regulation-ball tennis, soft-ball tennis, badminton, squash, etc., a method for producing the string, and a racket strung with the string.

BACKGROUND ART

Conventionally, a monofilament and/or multifilament string made of a synthetic fiber such as polyamide or polyester has been used widely as a racket string for tennis, badminton, squash, etc. In many cases, the conventional string mainly has a circular cross section and rather takes advantage of the features of the synthetic fiber to improve the durability.

Patent Documents 1 to 4 propose that the cross section of the string be deformed into an ellipse, an oval, a rectangle, or the like. Patent Document 1 proposes deforming the cross section during manufacture of the filament by using a rectangular spinneret for melt spinning. Patent Document 2 also proposes deforming the cross section during manufacture of the filament by using a spinneret with a dumbbell-shaped irregular cross section for melt spinning. Patent Document 3 proposes a string obtained by forming a core yarn to have a thick width and a thin width in advance and wrapping a sheath yarn around the core yarn. Patent Document 4 also proposes a string obtained by forming a core yarn to have an elliptical cross section in advance and wrapping a sheath yarn around the core yarn.

However, in the conventional technology, the cross section of the filament is deformed during melt spinning. Therefore, a twist or torsion can be applied in the subsequent processes such as wrapping of the sheath yarn and melt coating, and the wrapping is likely to be non-uniform or unstable. Moreover, Patent Documents 2 and 3 disclose strings that are subjected to a twisting (torsion) process. In these documents, the strings are twisted uniformly. If such strings are strung on rackets, the flat surfaces of the strings are distributed uniformly. Thus, the features of the flattened string cannot be used effectively.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 2009-50660 A -   Patent Document 2: JP 2005-348851 A -   Patent Document 3: JP 2000-210396 A -   Patent Document 4: JP S60 (1985)-77776 A

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

To solve the above conventional problems, the present invention provides a racket string that can make full use of the features of the flat surface, a method for producing the racket string, and a racket strung with the racket string.

Means for Solving Problem

A racket string of the present invention includes a synthetic fiber. The cross section is flattened by heating, compressing, and deforming the string after it has been formed, and an indentation is made in any part of the string. The flattened string is wound in a substantially non-twisted state.

A method for producing a racket string of the present invention produces the above racket string and includes the following: heating the string at a temperature of Tg (° C.) or more and Tm −10 (° C.) or less, where Tg (° C.) represents a glass transition point and Tm (° C.) represents a melting point of the main synthetic fiber constituting the string; compressing and deforming the string between rollers that are arranged with a predetermined clearance therebetween; and then cooling and winding up the string.

A racket of the present invention is strung with the above racket string. A principal surface of a hitting surface of the racket is formed of the flat surfaces of the strings, and the strings include uneven portions due to torsion that are present partially and non-uniformly on the hitting surface of the racket.

EFFECTS OF THE INVENTION

The racket string of the present invention includes a synthetic fiber, and the cross section is flattened by heating, compressing, and deforming the string after it has been formed, and an indentation is made in any part of the string. The flattened string is wound in a substantially non-twisted state. When this string is strung on a racket, the flat surfaces of the strings form the principal surface of the racket, and the uneven portions of the strings due to torsion are present partially and non-uniformly. Thus, the present invention can make full use of the features of the flat surface. Specifically, the present invention can improve the resiliency that allows a player to hit a ball or shuttlecock better and the spinnability that allows a player to create spin easily by rotation. Moreover, the softness also can be improved.

The method for producing the racket string of the present invention includes heating the string at a temperature of Tg (° C.) or more and Tm −10 (° C.) or less, where Tg (° C.) represents a glass transition point and Tm (° C.) represents a melting point of the main synthetic fiber constituting the string, compressing and deforming the string between rollers that are arranged with a predetermined clearance between them, and then cooling and winding up the string. With this method, the flattened string can be produced efficiently.

In the racket of the present invention, the principal surface of the hitting surface of the racket is formed of the flat surfaces of the strings, and the strings include uneven portions due to torsion that are present partially and non-uniformly on the hitting surface of the racket. With this configuration, the resiliency, the softness, and the spinnability can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a flattened string of an example of the present invention.

FIG. 2 is a diagram for explaining the production processes of a flattened string of an example of the present invention.

FIG. 3 is a cross-sectional view of a string before flattening in an example of the present invention.

FIG. 4 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 1 (clearance: 1.1 mm) of the present invention.

FIG. 5 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 1 (clearance: 1.0 mm) of the present invention.

FIG. 6 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 1 (clearance: 0.9 mm) of the present invention.

FIG. 7 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 2 (clearance: 1.1 mm) of the present invention.

FIG. 8 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 2 (clearance: 1.0 mm) of the present invention.

FIG. 9 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 3 (clearance: 1.0 mm) of the present invention.

FIG. 10 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 3 (clearance: 0.9 mm) of the present invention.

FIG. 11 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 4 (clearance: 1.0 mm) of the present invention.

FIG. 12 is a plan view of a racket for regulation-ball tennis that is strung with the flattened string produced in Example 4 (clearance: 0.9 mm) of the present invention.

DESCRIPTION OF THE INVENTION

The present invention relates to a string including a synthetic fiber. The synthetic fiber is not particularly limited. Examples of the synthetic fiber include the following: aliphatic or semiaromatic thermoplastic polyamides or copolymers thereof such as nylon 6, nylon 66, nylon 46, nylon 610, nylon 612, nylon 11, nylon 12, nylon 9T, and nylon 6T; thermoplastic polyesters or copolymers such as polyethylene terephthalate, polybutylene terephthalate, and polytrimethylene terephthalate; and aliphatic polyesters such as polylactic acid. In particular, the aliphatic or semiaromatic thermoplastic polyamides or copolymers thereof and the thermoplastic polyesters or copolymers are preferred, and the nylons and the thermoplastic polyesters are more preferred. This is because the nylons and the thermoplastic polyesters are suitable in terms of strength, elongation, and cost.

The configuration of the string is not particularly limited. The string can be in any form such as a monofilament alone, a multifilament, or a monofilament core wrapped with sheath yarns. The string is formed preferably by wrapping sheath yarns around a core, and more preferably by wrapping sheath yarns (thin monofilament yarns) around a monofilament core. It is preferable that a resin binder is used to join these fibers together. Moreover, a coating resin may be used. For example, a monofilament-type string is preferred that has a configuration in which a monofilament core is wrapped with sheath yarns (thin monofilament yarns), and the core and the sheath yarns are joined and then coated with a coating resin.

The racket string of the present invention is characterized by the cross section that is flattened by heating, compressing, and deforming the string after it has been formed. An oven or a dryer can be used for heating. The heating temperature is preferably Tg (° C.) or more and Tm −10 (° C.) or less, where Tg (° C.) represents a glass transition point and Tm (° C.) represents a melting point. This temperature may be an ambient temperature. After heating at the above temperature, the string is compressed and deformed, e.g., between rollers that are arranged with a predetermined clearance between them. As a result of these processes, an indentation is made in any part of the string.

The flattened string is wound up in a substantially non-twisted state. The term “substantially non-twisted” means that the number of twists is 5 turns or less per 1 meter. The preferred number of twists is zero. The string can be wound up in the substantially non-twisted state by a winder having a winding axis that is in the same direction or parallel to a pair of rollers. For commercial purposes, the string can be used as it is or formed into a skein. For personal purposes, the string may be cut to about 10 to 13 m, rewound in the substantially non-twisted state, and put in a package. While Patent Document 3 discloses a twisted string with a flat cross section, the present inventors found out that if the degree of flatness was 1.1 or more, even a non-twisted string underwent torsion when it was stretched on a racket by the usual stringing operation. In the context of the present invention, the term “torsion” means that the long diameter of the string that is substantially in a vertical position is twisted about 90 degrees to a horizontal position with respect to the racket surface, and also that the long diameter of the string in the horizontal position is turned to the vertical position. In some cases, the string may be twisted about 180 degrees. In the present invention, it is preferable that the racket surface includes approximately 10 to 100 such twisted portions. Using the string of the present invention, when gut is strung with the usual stringing method, the number of the twisted portions is increased as the degree of flatness becomes larger. The reasons the torsion occurs during stringing despite the use of a non-twisted string are considered to be as follows: (1) a so-called unwinding twist is given to the string as it is uncoiled; (2) the end of the string in a torsional state is put in a grommet and fixed by pulling the string without removing the torsion; and (3) when tension is applied to the intersection of a longitudinal string and a lateral string, the strings are twisted due to the tension. If the number of the twisted portions is 10 or more, the spinnability is improved. If the number of the twisted portions is 100 or less, the torsion is distributed non-uniformly over the entire racket surface.

In a racket that is strung with the string of the present invention, a principal surface of a hitting surface of the racket is formed of the flat surfaces of the strings, and the strings include uneven portions due to torsion that are present partially and non-uniformly on the hitting surface of the racket. Although the string of the present invention is not substantially twisted, an unwinding twist is given to the string as it is unwound from a take-up reel, or a twist and/or torsion is applied inevitably to the string as it is strung on a racket. Consequently, the flat surfaces of the strings form the principal surface, and the uneven portions of the strings due to torsion are present partially and non-uniformly. In this case, the principal surface is 50% or more of the surface for hitting a ball or shuttlecock (hitting surface).

Since the principal surface is formed of the flat surfaces of the strings, the contact area with a ball or shuttlecock is increased, and thus the resiliency is improved. Moreover, in the uneven portions due to torsion that are present partially and non-uniformly, the long side of the flattened string faces the hitting surface, so that it is easy to put spin on a ball. As described above, 10 to 100 uneven portions due to torsion are present preferably.

The string of the present invention is superior in playability to the string that is twisted uniformly by a twisting process. For example, when the string of the present invention is strung on a racket, the long diameter of the string is likely to be perpendicular to the racket surface in a bend such as near a grommet because of flexural rigidity. On the other hand, in a portion where the lateral string is woven, the string is rotated around the axis, and both the longitudinal string and the lateral string are substantially parallel to the racket surface. That is, the string tends to be perpendicular to the racket surface near the grommet, while it tends to be parallel on the racket surface. In the present invention, the racket surface further includes torsion, i.e., a twisted portion in which the longitudinal or lateral string is perpendicular. When the torsion is applied to the string, the long diameter of the string is turned to the vertical position with respect to the racket surface, thereby providing an uneven structure. This uneven portion is the feature when the string of the present invention is strung on a racket, and serves to improve the resiliency, the softness, and the spinnability.

On the other hand, the string that has been subjected to the twisting process is twisted periodically beforehand, and the number of twists also is large. Therefore, the string is not always perpendicular even near the grommet. Moreover, various twisted states are mixed together on the racket surface including the intersections of the longitudinal strings and the lateral strings, so that the twists as a whole are equalized or made uniform. Accordingly, there is a tendency to deny the features of the flattened string. Thus, it is not possible for the twisted string to achieve the excellent effects of the string of the present invention.

The ratio of the major axis to the minor axis (degree of flatness) of the cross section of the flattened string is preferably 1.1 to 1.8. Within this range, the resiliency and the spinnability can be improved further. Specifically, if the degree of flatness is 1.1 or more, the number of the twisted portions is increased after stringing of a racket, and the resiliency is improved significantly. If the degree of flatness is 1.8 or less, the stretchability of the string on the racket is no problem, since the degree of flatness of the string is proper. The degree of flatness is more preferably 1.2 to 1.7. In the present invention, it is preferable that a string before being compressed between the rollers has a substantially circular cross section with a degree of flatness of less than 1.1. This is because the string having such a degree of flatness can be formed to the same shape regardless of the angle from which it is compressed and deformed.

It is preferable that the string includes a core yarn composed of a monofilament, sheath yarns wrapped around the core yarn, and a coating resin joining the core yarn and the sheath yarns together, and that the core yarn mainly is flattened.

A method for producing a racket string of the present invention includes the following: heating the string at a temperature of Tg (° C.) or more and Tm −10 (° C.) or less, where Tg (° C.) represents a glass transition point and Tm (° C.) represents a melting point of the main synthetic fiber constituting the string; compressing and deforming the string between rollers that are arranged with a predetermined clearance between them; and then cooling and winding up the string. The predetermined clearance between the rollers is 0.8 to 1.2 mm, e.g., when the supplied string has a cross-sectional diameter of 1.30 to 1.35 mm. It is desirable that the clearance generally is set to be about 0.55 to 0.90 times the cross-sectional diameter of the supplied string, so that the minor axis of the resultant string is about 0.65 to 0.95 times the cross-sectional diameter of the formed string. The main synthetic fiber constituting the string indicates either a core yarn in the case of the presence of the core yarn or a synthetic fiber of 50 mass % or more in the case of the absence of the core yarn.

Hereinafter, the string will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a flattened string of an example of the present invention. A core yarn (monofilament) 1 made of nylon and located in the center is deformed largely into an ellipse, and sheath yarns (wrap yarns) 2 made of nylon and located around the core yarn 1 are deformed accordingly. It is observed that some portions of the sheath yarns penetrate into the core yarn. These deformation and penetration are indentations. Such indentations are made by compression molding with the rollers. A coating resin 3 covering the core yarn 1 and the sheath yarns 2 maintains substantially the same thickness. The coating resin 3 is preferably a nylon resin or the like. As shown in FIG. 1, the cross section of the flattened string includes (1) a deformation of the core yarn, (2) deformations of the sheath yarns, and (3) deformations of the contact portions between the core yarn and the sheath yarns (where the core yarn mainly is depressed). An embossed pattern may be formed on the roller surface. When the compression molding is performed with the rollers having the embossed pattern, the embossed pattern can be transferred to the surface of the resultant string.

FIG. 2 is a diagram for explaining the production processes for a flattened string of an example of the present invention. FIG. 3 is a cross-sectional view of a string before flattening. As shown in FIG. 2, a string 5 before processing, as shown in FIG. 3, is supplied from a feed reel 4 to a heating device 6, passes between rollers 7 a and 7 b with a predetermined clearance so that the string is deformed into a flat shape, and then is wound up on a take-up reel 11. In this manner, a substantially non-twisted flattened string 10 is provided. In FIG. 3, reference numeral 12 denotes a core yarn, 13 denotes sheath yarns, and 14 denotes a coating resin.

Compared to the method for producing a general string with a circular cross section, the method for producing the (flattened) string of the present invention further includes the flattening process, and therefore the number of processes may be increased. However, such an increase in the number of processes can be prevented by performing the flattening process simultaneously with other processes such as resin finish, oil application, and ink jet printing in the method for producing the general string with a circular cross section.

The method for producing the string of the present invention has the following features.

(1) The method of the present invention has high processing stability and can be used in a wide range of applications. In the method of the present invention, the cross section of the string is flattened by heating, compressing, and deforming the string. Therefore, when the string is processed under the proper conditions, the processing can be stable (without variation) in terms of the cross-sectional shape, the physical properties, the appearance, or the like. On the other hand, in a method for producing a flattened string, e.g., by wrapping sheath yarns (monofilaments) around an elliptical monofilament, since the curvature of the major axis differs from that of the minor axis in the core, the wrapped state becomes unstable. Thus, this method has the disadvantage of forming a gap between the sheath yarns or causing an overlap (protrusion) of the yarns. In many cases, the hole of a nozzle should be elliptical in the coating process of the flattened string. However, if torsion is applied to the string, it is likely that the coating resin is scraped or the yarns are broken while the string passes through the nozzle. Accordingly, it is difficult to produce a core yarn having a high degree of flatness stably with the conventional method.

(2) The playability is good. As a result of trial hitting actually conducted by advanced amateurs, the racket that was strung with the string of the present invention was chosen as a favorable string. In particular, the resiliency and the spinnability were rated highly by the advanced amateurs. Moreover, the resultant data also confirmed that high resiliency was obtained by a drop rebound test of an object after stringing, and that the friction between the ball and the string was increased due to flattening. Therefore, the actual spinnability is considered to be the result of the effect of the uneven portions on the racket surface.

EXAMPLES

Hereinafter, the present invention will be described in more detail by way of examples. However, the present invention is not limited to the following examples.

Example 1 Comparative Example 1

One monofilament (core yarn) that was made of nylon 6 and had a diameter of 1 mm and twenty-five monofilaments (sheath yarns) that were made of nylon 6 and had a diameter of 0.115 mm were prepared. An adhesive was applied to the surface of the core yarn, and then the sheath yarns were wrapped around the core yarn at 80 turns per meter (the number of twists). Subsequently, the core yarn and the sheath yarns were melt-coated with nylon 66 (coating resin). The resultant string 5 (FIG. 3) had a diameter of 1.30 to 1.31 mm (circular cross section). The main synthetic fiber (nylon 6) of the string had a glass transition point (Tg) of 40° C. and a melting point (Tm) of 215° C. This string 5 was defined as Comparative Example 1.

Next, as shown in FIG. 2, the string 5 was supplied to the heating device 6 and heated at 80° C. for 0.5 minutes. Then, the string 5 passed between the rollers 7 a and 7 b with a clearance (also referred to as a clearance gauge) of 0.9 mm, 1.0 mm, or 1.1 mm, and thus was deformed into a flat shape, resulting in the string 10 with a flat cross section as shown in FIG. 1. The ratio of the major axis to the minor axis (degree of flatness) of the cross section of each of the flattened strings was 1.67 for a clearance of 0.9 mm, 1.44 for a clearance of 1.0 mm, and 1.28 for a clearance of 1.1 mm. These strings were defined as Example 1.

FIGS. 4 to 6 show rackets for regulation-ball tennis that were strung with the resultant flattened strings. FIG. 4 shows the racket strung with the string obtained with a clearance of 1.1 mm. FIG. 5 shows the racket strung with the string obtained with a clearance of 1.0 mm. FIG. 6 shows the racket strung with the string obtained with a clearance of 0.9 mm. In FIGS. 4 to 6, black marks indicate the uneven portions of the strings due to torsion that are present partially and non-uniformly. On the other hand, no black mark indicates a region where the flat surfaces of the strings form the principal surface. The number of the uneven portions due to torsion is 22 in FIG. 4, 58 in FIG. 5, and 68 in FIG. 6.

The racket strung with the string having a degree of flatness of 1.44 was used by five tennis players. Then, all the tennis players evaluated that both the resiliency and the spinnability were higher compared to the string with a circular cross section (Comparative Example 1).

Example 2 Comparative Example 2

A monofilament that was made of polyethylene terephthalate blended with 5 mass % of polyethylene and had a diameter of 1.31 mm was used. The main synthetic fiber (polyethylene terephthalate) of the string had a glass transition point (Tg) of 70° C. and a melting point (Tm) of 260″C. This string was defined as Comparative Example 2.

Next, as shown in FIG. 2, the string 5 was supplied to the heating device 6 and heated at 80° C. for 0.5 minutes. Then, the string 5 passed between the rollers 7 a and 7 b with a clearance (also referred to as a clearance gauge) of 1.0 mm or 1.1 mm, and thus was deformed into a flat shape, resulting in the string with a flat cross section. The ratio of the major axis to the minor axis (degree of flatness) of the cross section of each of the flattened strings was 1.72 for a clearance of 1.0 mm and 1.47 for a clearance of 1.1 mm. These strings were defined as Example 2.

FIGS. 7 to 8 show rackets for regulation-ball tennis that were strung with the resultant flattened strings. FIG. 7 shows the racket strung with the string obtained with a clearance of 1.1 mm. FIG. 8 shows the racket strung with the string obtained with a clearance of 1.0 mm. In FIGS. 7 to 8, black marks indicate the uneven portions of the strings due to torsion that are present partially and non-uniformly. On the other hand, no black mark indicates a region where the flat surfaces of the strings form the principal surface. The number of the uneven portions due to torsion is 13 in FIG. 7 and 25 in FIG. 8.

The racket strung with the string having a degree of flatness of 1.47 was used by five tennis players. Then, all the tennis players evaluated that both the resiliency and the spinnability were higher compared to the string with a circular cross section (Comparative Example 2).

Example 3 Comparative Example 3

Two types of three-island monofilaments having a circular cross section were prepared as a core yarn with a diameter of 0.89 mm and a sheath yarn with a diameter of 0.195 mm. Each of the monofilaments includes three island components including nylon 6 blended with 10 mass % of polyethylene terephthalate and a sea component including nylon 6. Sixteen three-island monofilaments (sheath yarns) were wrapped around one three-island monofilament (core yarn) at 85 turns per meter (the number of twists). Subsequently, the core yarn and the sheath yarns were coated with nylon 6 (coating resin). The resultant string had a diameter of 1.32 mm. The main synthetic fiber (nylon 6) of the string had a glass transition point (Tg) of 40° C. and a melting point (Tm) of 215° C. This string was defined as Comparative Example 3.

Next, as shown in FIG. 2, the string 5 was supplied to the heating device 6 and heated at 80° C. for 0.5 minutes. Then, the string 5 passed between the rollers 7 a and 7 b with a clearance (also referred to as a clearance gauge) of 0.9 mm or 1.0 mm, and thus was deformed into a flat shape, resulting in the string with a flat cross section. The ratio of the major axis to the minor axis (degree of flatness) of the cross section of each of the flattened strings was 1.42 for a clearance of 0.9 mm and 1.28 for a clearance of 1.0 mm. These strings were defined as Example 3.

FIGS. 9 to 10 show rackets for regulation-ball tennis that were strung with the resultant flattened strings. FIG. 9 shows the racket strung with the string obtained with a clearance of 1.0 mm. FIG. 10 shows the racket strung with the string obtained with a clearance of 0.9 mm. In FIGS. 9 to 10, black marks indicate the uneven portions of the strings due to torsion that are present partially and non-uniformly. On the other hand, no black mark indicates a region where the flat surfaces of the strings form the principal surface. The number of the uneven portions due to torsion is 81 in FIGS. 9 and 91 in FIG. 10.

The racket strung with the string having a degree of flatness of 1.42 was used by five tennis players. Then, all the tennis players evaluated that both the resiliency and the spinnability were higher compared to the string with a circular cross section (Comparative Example 3).

Example 4 Comparative Example 4

One monofilament (core yarn) that was made of nylon 6 and had a diameter of 1 mm and twenty-three monofilaments (sheath yarns) that were made of nylon 6 and had a diameter of 0.14 mm were prepared. An adhesive was applied to the surface of the core yarn, and then the sheath yarns were wrapped around the core yarn at 80 turns per meter (the number of twists). Subsequently, the core yarn and the sheath yarns were coated with nylon 66 (binder coating resin). The resultant string 5 had a diameter of 1.30 to 1.31 mm (circular cross section). The main synthetic fiber (nylon 6) of the string had a glass transition point (Tg) of 40° C. and a melting point (Tm) of 215° C. This string was defined as Comparative Example 4.

Next, as shown in FIG. 2, the string 5 was supplied to the heating device 6 and heated at 80° C. for 0.5 minutes. Then, the string 5 passed between the rollers 7 a and 7 b with a clearance (also referred to as a clearance gauge) of 0.9 mm or 1.0 mm, and thus was deformed into a flat shape, resulting in the string with a flat cross section. The ratio of the major axis to the minor axis (degree of flatness) of the cross section of each of the flattened strings was 1.49 for a clearance of 0.9 mm and 1.36 for a clearance of 1.0 mm. These strings were defined as Example 4.

FIGS. 11 to 12 show rackets for regulation-ball tennis that were strung with the resultant flattened strings. FIG. 11 shows the racket strung with the string obtained with a clearance of 1.0 mm. FIG. 12 shows the racket strung with the string obtained with a clearance of 0.9 mm. In FIGS. 11 to 12, black marks indicate the uneven portions of the strings due to torsion that are present partially and non-uniformly. On the other hand, no black mark indicates a region where the flat surfaces of the strings form the principal surface. The number of the uneven portions due to torsion is 32 in FIG. 11 and 47 in FIG. 12.

The racket strung with the string having a degree of flatness of 1.36 was used by five tennis players. Then, all the tennis players evaluated that both the resiliency and the spinnability were higher compared to the string with a circular cross section (Comparative Example 4).

Comparative Example 5

One monofilament (core yarn) that was made of nylon 6 and had an elliptical cross section with a major axis of 1.23 mm and a minor axis of 0.75 mm and twenty-five monofilaments (sheath yarns) that were made of nylon 6 and had a diameter of 0.115 mm were prepared. Using an adhesive in which nylon 6 was dissolved in a phenol solvent, the sheath yarns were wrapped around the core yarn at 80 turns per meter (the number of twists), and then bonded and dried. The cross section of the resultant string had a major axis of 1.45 mm and a minor axis of 1.03 mm (ellipse). The ratio of the major axis to the minor axis (degree of flatness) of the cross section of the string was 1.45. The main synthetic fiber (nylon 6) of the string had a glass transition point (Tg) of 40° C. and a melting point (Tm) of 215° C. At this stage, the adhesion of the core and the sheath yarns was non-uniform, and there was a large gap between the sheath yarns in the flat portion. This is because, due to the flat elliptical cross section of the core, the application of the adhesive, the pressure bonding between the core and the sheath yarns, and the wrapping angle can be non-uniform and unstable.

Subsequently, the above string was coated with nylon 66 (binder coating resin) with a nozzle having an elliptical cross section. The cross section of the coated string had a major axis of 1.47 mm and a minor axis of 1.08 mm (ellipse). The ratio of the major axis to the minor axis (degree of flatness) of the cross section of the coated string was 1.36. The thickness of the coating of the coated string was non-uniform.

The coated string was strung on a racket for regulation-ball tennis. When the coated string was used, fuzzing or partial peeling occurred, and one of three strings was broken.

This racket was used by five tennis players. Then, all the tennis players evaluated that both the resiliency and the spinnability were lower compared to the strings in Example 1, and that fuzzing and peeling were likely to occur.

With respect to Example 1 and Comparative Example 1, Table 1 shows the processing conditions (the clearance, the heating temperature and time, and the winding speed), the cross section of the resultant string (the dimensions and the degree of flatness), the physical properties of the resultant string (the strength, the elongation, the intermediate elongation, and the S knot strength), and the properties when the resultant string was strung on a racket (the stretchability, the number of twisted portions, and the playability).

TABLE 1 Comparative Example 1 (before flattening) Example 1 Processing Clearance (mm) — 1.1 1.0 0.9 conditions Heating temperature and time — 80° C., 0.5 min Winding speed (m/min) — 4 Cross Dimensions (mm) Minor axis 1.30 1.12 1.03 0.94 section Major axis 1.31 1.43 1.48 1.57 Degree of flatness Major axis/Minor axis 1.01 1.28 1.44 1.67 Physical Strength (kg) 83.7 80.7 81.0 80.5 properties Elongation (%) 34.1 32.5 31.1 33.5 Intermediate elongation (at 23 kg, %) 10.3 10.8 9.5 10.5 S knot strength (kg) 43.5 40.6 40.0 40.0 Gut Stretchability A A A A Properties Number of twisted portions 0 22 58 68 Playability Resiliency (%) 62 64 65 66 Spinnability (r.p.m) 250 330 390 400

In Table 1, the strength was measured in accordance with JIS L 1013. The elongation was measured in accordance with JIS L 1013. The intermediate elongation was measured in accordance with JIS L 1013. The S knot strength was measured in accordance with JIS L 1013. For the stretchability, “A” indicates that “the string can be strung on a racket without any problem”, “B” indicates that “the string can be strung on a racket, but some portions of the string do not easily pass through a grommet”, and “C” indicates that “the string does not pass through a grommet and cannot be strung on a racket”.

The resiliency was determined in the following manner. A weight of 4 kg was dropped onto a stretched gut surface from a height of 200 mm, and the rebound height (H) of the bouncing weight was measured. Then, the resiliency was calculated by the following formula.

Resiliency (%)=(H/200)×100

The spinnability was determined in the following manner. A ball rotating at 850 rpm (R1) was dropped onto a stretched gut (single) from a height of 400 mm, and the number of revolutions (R2) was measured 10 ms after the ball hit the gut surface. Then, the spinnability was calculated by the following formula. When the value of the spinnability is large, it means that the friction between the ball and the gut is large, which makes it possible to put more spin on the ball.

Spinnability (rpm)=R1−R2

As shown in Table 1, the results of Example 1 and Comparative Example 1 confirmed that the resiliency and the spinnability of the string of this example were improved.

Example 5 Comparative Example 6

A high-strength nylon 6 multifilament (manufactured by TORAY INDUSTRIES, INC., trade name: “Amilan”, composed of three pieces of 2100T-306f and a piece of 940T-136f) was impregnated with a UV curable resin, squeezed through a nozzle (1.1 mmφ), twisted at 300 T/m, and hardened by UV irradiation, thus providing a core yarn. Then, the core yarn was melt-coated with nylon 6, so that a multifilament-type string with a circular cross section was produced. The string had a diameter of 1.23 mm, a strength of 64 kg, an elongation of 24%, and a knot strength of 27 kg. This string was defined as Comparative Example 6.

Next, the string was heated at 100° C. and flattened by passing between the rollers with a clearance of 1.10 mm. The flattened string had a major axis of 1.44 mm, a minor axis of 1.12 mm, and a degree of flatness of 1.29. Moreover, the flattened string had a strength of 64 kg, an elongation of 26%, and a knot strength of 29 kg. The resultant string was defined as Example 5.

Subsequently, this string was strung on a racket for regulation-ball tennis at a tension of 60 pounds. The number of the twisted portions on the racket surface was 25. For comparison, the string before flattening was strung on the same racket, and then the two rackets were compared by trial hitting. Consequently, four of five players evaluated that the flattened string of this example created more spin than the string before flattening. To evaluate the spinnability, the number of revolutions of the ball when it bounced back from the racket was measured with a high-speed camera. At the time of hitting, the ball speed was 100 km/h, the incident angle was 40 degrees (with respect to the vertical direction), and the number of revolutions was approximately 0 rpm. The number of revolutions of the ball that bounced back from the racket was measured five times, and the average value of the five measurements was 3060 rpm for the flattened string of this example, while the average value was 2930 rpm for the string with a circular cross section. The results showed that the use of the flattened string of this example increased the number of revolutions of the ball and created more spin.

Example 6 Comparative Example 7

Using a polyethylene terephthalate resin (I V=1.1), a string was produced by flattening a monofilament with a circular cross section, and another string having substantially the same flat cross section was produced by spinning and drawing using a substantially rectangular spinneret under the same conditions. Then, these strings were compared.

The monofilament with a circular cross section having a diameter of 1.29 mm was heated at 150° C. and flattened by passing between the rollers with a clearance of 1.1 mm. The flattened string had a major axis of 1.45 mm, a minor axis of 1.12 mm, a degree of of 1.29, a strength of 61 kg, an elongation of 34%, and a knot strength of 44 kg (Example 6). On the other hand, the comparative flattened string produced using the rectangular spinneret had a major axis of 1.42 mm, a minor axis of 1.13 mm, a degree of flatness of 1.26, a strength of 66 kg, an elongation of 31%, and a knot strength of 49 kg (Comparative Example 7).

The two types of strings were strung on the same type of racket at a tension of 60 pounds. The number of twisted portions was 18 in the racket of this example and 16 in the racket of the comparative example. These rackets were used for trial hitting by five players, and four of them evaluated that the racket of this example was softer and had better bite onto the ball.

In the drop rebound test used for evaluating the resiliency in Example 1, the amount of depression of the weight when it hit the racket surface was measured. Consequently, the maximum depression was 9 mm for the racket of this example, while the maximum depression was 6 mm for the racket of the comparative example. The results showed that the amount of depression was larger in the racket of this example, i.e., the racket of this example was softer and had better bite onto the ball than that of the comparative example. Moreover, the spinnability also was evaluated in the above manner. At the time of hitting, the ball speed was 100 km/h, the incident angle was 40 degrees, and the number of revolutions was approximately 0 rpm. The number of revolutions of the ball that bounced back from the racket was 3130 rpm for the flattened string of the racket of this example, while the number of revolutions of the ball was 2900 rpm for the string with a circular cross section. The results showed that the use of the flattened string of this example increased the number of revolutions of the ball and created more spin.

Although the reason for such differences is not clear, a change in the fine structure in the thickness direction of the cross section caused by heating and deforming the string during the flattening process may affect the differences.

INDUSTRIAL APPLICABILITY

The string of the present invention is useful for rackets of regulation-ball tennis, soft-ball tennis, badminton, squash, etc.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1, 12 Core yarn (monofilament)     -   2, 13 Sheath yarn (wrap yarn)     -   3, 14 Coating resin     -   4 Feed reel     -   5 String before processing     -   6 Heating device     -   7 a, 7 b Roller     -   10 Flattened string     -   11 Take-up reel 

1. A racket string comprising a synthetic fiber, wherein a cross section is flattened by heating, compressing, and deforming the string after it has been formed, and an indentation is made in any part of the string, and wherein the flattened string is wound in a substantially non-twisted state.
 2. The racket string according to claim 1, wherein a ratio of a major axis to a minor axis (degree of flatness) of the cross section of the flattened string is 1.1 to 1.8.
 3. The racket string according to claim 1, wherein the synthetic fiber is at least one selected from a thermoplastic polyamide and a thermoplastic polyester.
 4. The racket string according to claim 1, wherein the string comprises a core yarn composed of a monofilament, sheath yarns wrapped around the core yarn, and a coating resin joining the core yarn and the sheath yarns together, and wherein the core yarn mainly is flattened.
 5. The racket string according to claim 4, wherein the monofilament of the core yarn is a thermoplastic polyamide.
 6. The racket string according to claim 1, wherein the synthetic fiber is a thermoplastic polyester, and the string has a structure that includes a monofilament alone or a monofilament with coating.
 7. A method for producing a racket string comprising a synthetic fiber, the method comprising: heating the string at a temperature of Tg (° C.) or more and Tm −10 (° C.) or less, where Tg (° C.) represents a glass transition point and Tm (° C.) represents a melting point of a main synthetic fiber constituting the string; compressing and deforming the string between rollers that are arranged with a predetermined clearance therebetween; and then cooling and winding up the string, wherein a cross section is flattened by heating, compressing, and deforming the string after it has been formed, and an indentation is made in any part of the string, and wherein the flattened string is wound in a substantially non-twisted state.
 8. The method according to claim 7, wherein the clearance between the rollers for compressing and deforming the string is 0.55 to 0.90 times a string diameter before the compression, so that a minor axis of the flattened string is 0.65 to 0.95 times the string diameter before the compression.
 9. A racket strung with a racket string comprising a synthetic fiber, wherein a cross section is flattened by heating, compressing, and deforming the string after it has been formed, and an indentation is made in any part of the string, and wherein the flattened string is wound in a substantially non-twisted state, wherein a principal surface of a hitting surface of the racket is formed of the flat surfaces of the strings, and the strings include uneven portions due to torsion that are present partially and non-uniformly on the hitting surface of the racket.
 10. The racket according to claim 9, wherein the number of the uneven portions due to torsion is 10 to
 100. 11. The method according to claim 8, wherein a ratio of a major axis to the minor axis (degree of flatness) of the cross section of the flattened string is 1.1 to 1.8.
 12. The method according to claim 8, wherein the synthetic fiber is at least one selected from a thermoplastic polyamide and a thermoplastic polyester.
 13. The method according to claim 8, wherein the string comprises a core yarn composed of a monofilament, sheath yarns wrapped around the core yarn, and a coating resin joining the core yarn and the sheath yarns together, and wherein the core yarn mainly is flattened.
 14. The method according to claim 13, wherein the monofilament of the core yarn is a thermoplastic polyamide.
 15. The method according to claim 8, wherein the synthetic fiber is a thermoplastic polyester, and the string has a structure that includes a monofilament alone or a monofilament with coating.
 16. The racket according to claim 9, wherein a ratio of a major axis to a minor axis (degree of flatness) of the cross section of the flattened string is 1.1 to 1.8.
 17. The racket according to claim 9, wherein the synthetic fiber is at least one selected from a thermoplastic polyamide and a thermoplastic polyester.
 18. The racket according to claim 9, wherein the string comprises a core yarn composed of a monofilament, sheath yarns wrapped around the core yarn, and a coating resin joining the core yarn and the sheath yarns together, and wherein the core yarn mainly is flattened.
 19. The racket according to claim 18, wherein the monofilament of the core yarn is a thermoplastic polyamide.
 20. The racket according to claim 9, wherein the synthetic fiber is a thermoplastic polyester, and the string has a structure that includes a monofilament alone or a monofilament with coating. 